Wildfire Defensible Space Calculator: Zone Areas, Tree and Shrub Spacing
Plan defensible space around a home: the area of Zone 0 (0–5 ft ember-resistant), Zone 1 (5–30 ft) and Zone 2 (30–100 ft) from your house footprint and property line, CAL FIRE tree-canopy and shrub spacing by slope, trees to keep and remove, Zone 0 gravel, upkeep hours and a home-hardening checklist.
How it works
area within d of the house = 2d(L + W) + πd² (d capped at the property line); Zone 0 = within 5 ft (1.52 m), Zone 1 = within 30 ft (9.14 m) − Zone 0, Zone 2 = within 100 ft (30.5 m) − within 30 ft · canopy gap = 10 ft (flat, under 20%), 20 ft (20–40%), 30 ft (over 40%) · shrub gap = 2×, 4× or 6× shrub height · clear height under branches = max(6 ft, 3 × shrub height) · trees per hectare ≤ 10,000 ÷ (crown + gap)² · gravel = Zone 0 area × depth
Most homes lost to wildfire are ignited by embers landing on something flammable next to the building, so defensible space is organised in rings. CAL FIRE and NFPA’s Firewise program use the same three: Zone 0, the first 5 ft, kept free of anything that burns; Zone 1, out to 30 ft, kept lean, clean and green; and Zone 2, out to 100 ft or the property line, where fuel is thinned so a fire drops from the crowns to the ground and slows. The calculator measures each ring around your footprint with rounded corners, so the distance from the walls is the same all the way round, and stops at your property line. Spacing comes from CAL FIRE’s plant-spacing guide. On flat ground or slopes under 20%, tree canopies should be 10 ft apart edge to edge and shrubs twice their height apart; on 20–40% slopes 20 ft and 4 times; above 40%, 30 ft and 6 times, because fire climbs slopes much faster. Branches are trimmed at least 6 ft up, and over shrubs to 3 times the shrub height, to break ladder fuels; grass in Zone 2 stays at 4 in or less. From the crown size and gap the calculator finds how many trees a hectare or acre can carry, compares that with your current canopy cover and estimates how many trees to remove in Zone 2. It also sizes the gravel for Zone 0 and gives a rough yearly upkeep time. Local ordinances may ask for more, and clearing works best together with ember-resistant roofs, vents, decks and windows.
Worked example
Default: an 18 × 12 m (59 × 39 ft) house on a 20–40% slope with the property line 40 m away. Zone 0 = 2 × 1.524 × (18 + 12) + π × 1.524² = 98.7 m²; Zone 1 = 712.6 m²; Zone 2 = 3,936.1 m²; 4,747 m² (51,101 ft²) in all. Canopies need a 20 ft (6.1 m) gap, so with 6 m crowns the pitch is 12.1 m and Zone 2 can carry about 68 trees per hectare (19% canopy). At 60% cover there are about 84 trees in Zone 2 now: keep about 27 and remove about 57 (about $22,650 at $400 a tree). 1 m shrubs need 4 m between them and 3 m of clear trunk above them. Zone 0 gravel at 7.5 cm: 7.4 m³, about 11.8 t. Yearly upkeep: 9–19 hours.
Sources: CAL FIRE Ready for Wildfire: defensible space zones and plant spacing · NFPA: preparing homes for wildfire (Firewise home ignition zones)
Frequently asked questions
What are the defensible space zones?
Zone 0 is 0–5 ft from the house and any attached deck: no combustible materials at all. Zone 1 is 5–30 ft: remove dead vegetation, space and limb up trees and shrubs. Zone 2 is 30–100 ft or to the property line: reduce fuel, keep grass short and space trees so fire cannot jump between crowns.
How far apart should trees be for fire safety?
CAL FIRE asks for at least 10 ft between tree canopies on flat ground or slopes under 20%, 20 ft on 20–40% slopes and 30 ft on slopes over 40%. The gap is measured edge to edge between crowns, not between trunks.
Do I have to remove all trees near my house?
No. Healthy, well-spaced trees with lower limbs trimmed at least 6 ft up can stay in Zones 1 and 2. What has to go is continuity: shrubs under trees, touching crowns, dead wood and anything combustible within 5 ft of the walls, plus branches within 10 ft of a chimney.
My lot is too small for 100 ft. What should I do?
Do everything you can inside your property line; Zones 0 and 1 give the biggest benefit per square foot. Required clearing stops at the property line, so talk to neighbours or join a Firewise or community fuel-reduction project for the rest.
What should go in Zone 0?
Gravel, pavers, concrete or bare mineral soil. Replace bark mulch, move firewood, lumber, trash bins and doormats away, keep gutters and the roof clear of needles and leaves, and enclose the area under decks.
Related calculators
Emergency-planning figures based on FEMA, Red Cross, CDC and extension-service guidance. Local hazards, official evacuation orders and building codes take precedence — this is a planning aid, not a substitute for them.